Fitting Together the H I Absorption and Emission in the Southern Galactic Plane Survey
J. M. Dickey, N. M. McClure-Griffiths, B. M. Gaensler, A. J. Green;
ApJ, 2003, 585, 801
ABSTRACT:In this paper we study 21 cm absorption spectra and the corresponding emission spectra toward bright continuum sources in the test region (326deg<l<333deg) of the Southern Galactic Plane Survey.This survey combines the high resolution of the Australia Telescope Compact Array with the full brightness temperature information of the Parkes single-dish telescope.
In particular, we focus on the abundance and temperature of the cool atomic clouds in the inner Galaxy.
The resulting mean opacity of the H I, <κ>, is measured as a function of Galactic radius; it increases going in from the solar circle, to a peak in the molecular ring of about 4 times its local value.
This suggests that the cool phase is more abundant there, and colder, than it is locally.
The distribution of cool-phase temperatures is derived in three different ways.
The naive, ``spin temperature'' technique overestimates the cloud temperatures, as expected.
Using two alternative approaches, we get good agreement on a histogram of the cloud temperatures, Tcool, corrected for blending with warm-phase gas.
The median temperature is about 65 K, but there is a long tail reaching down to temperatures below 20 K.
Clouds with temperatures below 40 K are common though not as common as warmer clouds (40-100 K).
Using these results, we discuss two related quantities, the peak brightness temperature seen in emission surveys and the incidence of clouds seen in H I self-absorption.
Both phenomena match what would be expected based on our measurements of <κ> and Tcool.
KEYWORDS: ism: atoms, ism: clouds, ism: structure, radio lines: ism
PERSOKEY:h_i, 21 cm, absorption, ,
CODE: dickey2003